Method for the production of high-concentration manganese mini-tablets for alloying aluminum baths and device for implementing said method

ABSTRACT

The method has the aim of obtaining Mn mini-tablets with a concentration of the metal ranging between 90 and 98%, Al particles being the binding element. The method is based on the use of ground electrolytic Mn from shales with a chemical purity of 99.7% or higher. The product is screened with a mesh of less than 450 micra, wherein the fine powder content should be less than 15%. Moreover, atomized powder Al obtained by mechanical processes with a granulometry of between 100 and 800 micra and with over 80% of the powder being between 350 and 720 micra should be used in the method. The method is carried out in a device having a storage hopper ( 1 ), a diffuser ( 4 ) of the product in the hopper ( 1 ), a hopper for compacting and shaping the mini-tablets in molds ( 9 ) in combination with pressing punches ( 7  and  8 ) and with the aid of an alveolar and dosing valve ( 10 ) mounted between the feed chamber ( 5 ) and the compacting chamber ( 6 ).

OBJECT OF THE INVENTION

The present invention refers to a procedure for the manufacture of highconcentration manganese (Mn) minitablets for aluminium (Al) bathalloying, the purpose of which is to produce Mn minitablets with a90-98% concentration of this metal, for adding in Al smelting.

The object of the invention is to produce a minitablet product composedof Mn and Al powder whose first component is obtained by electrolysisand grinding, while the second component is an atomized powder producedby means of mechanical processes, both components being them mixed andcompacted to form minitablets with a high Mn concentration.

A further object of the invention is the device for the execution of theabove-mentioned procedure, the device where the loading, dispensing,compacting and final forming of the minitablets take place.

BACKGROUND OF THE INVENTION

The alloying of aluminium baths with manganese has changed substantiallyin recent times, and from the original addition of lumps of metal, whichgave rise to serious problems of purity and dissolving rate, there hasbeen a shift towards two different concepts of alloying: on the onehand, the use of parent alloys, consisting of Al and Mn manganese alloyswith a 10 to 25% Mn content, and, on the other, the addition of powderedMn by means of injecting the powder into the furnace. Although bothmethods are still employed today, their use has declined drasticallysince the first compact Mn pellets were introduced towards the end ofthe seventies. These pellets, which came in the form of tablets,minitablets or briquettes, combine concepts of the two previous methods,take advantage of their strong points and reduce their drawbacks. Thepellets consist of Mn powder in a concentration usually above 75%compacted using Al powder as the binding agent, a flux, or a mixture ofboth, in a concentration of up to 25%. These materials substantiallyreduced the amount of cold material which is added to the Al furnace inthe alloying operation in comparison with parent alloys. Furthermore,parent alloys usually contain 75 to 90% second smelting aluminium, whichcould give rise to problems in the molten metal, besides calling for astock 4 times higher than that of compacted powder alloying agents.Moreover, they are easy-to-use materials that do not require theinvestment in equipment and safety that is necessary for powderinjection.

The great financial step that was taken on changing parent alloyscontaining a maximum of 25% Mn to compact alloying agents whose Mncontent is 75% or more has generated constant pressure on themanufacturers of compact alloying agents to obtain materials that, whilebeing effective in the Al bath alloying process, also succeed inincreasing the Mn concentration in the compact alloying agent. In thisrespect, no materials are available on the market that contain apercentage above 85% Mn, due mainly to the problems of compactibility ofMn, an abrasive and non-ductile material. In addition, it is suspectedthat the material may not dissolve as quickly as the compact materialsof lower Mn concentration due to the reduced proportion of Al and/orflux, which also act as disintegrators of the compact when this is putinto the furnace, as reported by the scientific literature on thesubject.

As the active alloying element of the compacts is Mn, the decreased Alcontent brings a series of advantages for the founder. The amount ofmaterial to be added to the furnace is smaller, which means that lesscold material is added to the Al bath and that raw material stocks arereduced. Similarly, there is a cut in material transport costs, whichwill be significantly lower than those of 75% or 80% compacts. Besidesthis, the price of products depends less on the value of Al, subject tothe changes in its quotation on the London Metal Exchange, and since Alis currently more expensive than Mn, the cost of the set of the rawmaterials used in production would also be lower. Lastly, we have toconsider that the founder/user is not interested in adding a material(Al powder) to his furnace that he is able to sell himself and which,moreover, has a value added due to atomisation, which is lost whensmelting it again.

Despite these financial advantages, no compact Mn materials haveappeared on the market with a concentration of 90% or more. Theattainment of this objective raises a series of scientific challengeswhen it comes to flow production of these materials. On the one hand,experience indicates that the pressing process has to be improved inorder to be able to reach these Mn percentages. On the other, the rawmaterials have a series of factors that may be modified when it comes toachieving a better performance. In addition, it has to be confirmedwhether it is really necessary to have compacts in the furnace with Alpowder concentrations of more than 10% or 15% for the Mn dissolving rateto be acceptable, or whether compacts with less than 10% Al dissolve inthe furnace at a suitable rate.

The present study concentrates on the flow production and performance inthe Al furnace of alloying minitablets (cylindrical in shape) containingMn in a concentration of more than 90%, Al being the remaining material.Although it would be desirable to have this concentration available instandard sized tablets as well, the need to apply high pressures to thematerial means that the study is complicated if the size of the compactdiameter is larger than 40 mm. On the other hand, fluxes were initiallyrejected in this study insofar as they are materials whose bindingaction is considerably inferior to that of Al powder.

With regard to the raw materials to be used, Mn is the first limitationof the study. The chemical requirements of Al baths involve the use ofMn of a high chemical purity, usually above the 99.7% level, which canonly be assured if the Mn is produced by electrolysis. At presentelectrolytic Mn is only produced in the Republic of South Africa and thePeople's Republic of China, which reduces the possibilities of findingmaterials with different specifications. Mn, which is usually in flakeform, has to be converted into powder by grinding. The material normallyused in the compacting of Mn minitablets has a grain size of less than450 microns. Mn powder is highly abrasive, a property that is enhancedif the amount of fines (powder below 100 microns) increases, and whichhas a direct effect on the pressing quality and the average life of thematerials (punches and liner) of the press in which the material iscompacted.

The situation is very different with regard to the Al powder involved.There is a great variety of Al powders on the market that may be used incontinuous industrial processes and with different applications. In thecase of compacting Mn half-tablets, it is normal to use Al fractionsabove 100 microns and below 1000 microns insofar as grain size isconcerned. These fractions are the ones that Al producers generallyregard as a by-product in their production processes, inasmuch as thefine fractions of Al (below 100 microns) are the ones that have valuableapplications in aeronautics and pyrotechnics on account of the explosiveproperty of Al. This fact means that again the production of a materialof specific characteristics for the compacting of Mn tablets is skewedor subject to production conditions independent of the application thatthis study sets out to examine.

In general, the Al used in the production of Mn minitablets is agas-atomized powder, although materials may be used that are obtained bymechanised atomisation procedures, annealed materials or micronisedswarf. As a rule, atomized powders are the most suited to therequirements of the main functions of Al.

In the production of the Mn minitablets, Al acts as a binding agent,whereas electrolytic Mn, being highly abrasive and non-ductile, is amaterial that does not compact on its own. Potential improvements in theprocess apparently lie in the application of higher pressures to thematerial so as to enable these materials to be compacted. Apart fromusing higher performance hydraulic units and applying greater force tothe pressing punches, another possibility is to reduce the diameter ofthe minitablets, as the smaller the area of application is, the greaterthe actual pressure. This represents a problem at industrial level, assmaller diameter minitablets give rise to lower productivity(minitablets weigh less). To overcome this problem, it is necessary towork with several punches at the same time, and the pressing process hasto be effective for all the minitablets made in a cycle. This means thatall the liners have to be filled properly with the material to becompacted, that this must be mixed properly and not be different in eachof the liners in which it is received, and that the material must flowsmoothly to these liners. In this respect, it is extremely important tostop the mixture of Mn and Al powders from becoming separated at anytime in the process (a problem that could arise easily since the twomaterials have widely differing densities) and, furthermore, that theequipment should be suitably sized so as to apply the pressure neededfor compacting.

DESCRIPTION OF THE INVENTION

The procedure that is advocated offers a solution to the problems anddifficulties mentioned in the previous section, for which purpose it isspecified that, starting from the two components used, which have to bemixed, namely Mn and Al, Mn minitablets with a concentration of morethan 90% should be compacted by using Mn produced by electrolysis andground from flakes of Mn of a chemical purity of 99.7% or more, which issubjected to a screening process with a sieve with a mesh of less than450 microns; the special feature of the Mn grinding process is that itis controlled so that the content of fine Mn powder, with a size of lessthan 100 microns, should not be more than 15%, as above this proportionthe compacting of Mn minitablets cannot be assured with over 90% Mn intheir composition.

The procedure also includes the fact that the most suitable Al forsuccessfully compacting Mn minitablets is atomized powder, which isproduced by mechanical processes, with controlled size distribution, itsnominal grain size intervals being between 100 and 800 microns, withover 80% of the powder in the 350-720 micron range.

This grain size distribution is coarse enough to enable the material tobe compacted and fine enough not to retard the dissolving rate, throughhaving reduced the number of Al grains with the increased Mnconcentration in the minitablet.

The invention also refers to the device for executing the foregoingprocedure, consisting of a hopper for the reception of an Mn and Al mixwith the afore-mentioned characteristics, there being a central productdiffuser in this hopper which forces the product to flow through thesides of the hopper to prevent the mix directly reaching the feeder of asecond hopper which discharges into the respective pressing orcompacting chamber, where pressing punches will come into action.

The device has appropriate means that enable maximum, minimum and safetylevels to be kept under control in the compacting chamber so that itremains at a level of filling all the time such that none of the punchesmay try and make an off-load compacting stroke.

As one of its main innovative features, besides the afore-mentionedcentral diffuser, the device includes a honeycomb dispensing valveinterposed between the feed hopper and the compacting chamber, which isprovided with a series of dies that are mounted on a support integralwith the actual feed hopper, so that the support-hopper assembly is ableto run along guides, in either direction, under the action of apneumatic device, on which guides there is in turn a moving punchsupport mounted, also driven by a pneumatic ram, so that thesupport-hopper movement is independent of the moving punch movement,although such movements must be synchronised in order to fill, press,compact and eject the formed minitablet.

Besides the aforesaid central diffuser and the location and use of thehoneycomb dispensing valve, as an innovative feature, the device alsoincludes three electrical control means to monitor the maximum, minimumand safety levels, corresponding to compacting chamber filling.

DESCRIPTION OF THE DRAWINGS

To supplement the description being given and in order to assist abetter understanding of the features of the invention, in accordancewith a preferred example of a practical embodiment of same, as anintegral part of this description a set of drawings is adjoined,wherein, for purely illustrative and non-restrictive purposes, thefollowing is represented:

FIG. 1.—It shows the graph corresponding to the standard grain sizedistribution of the Mn used in the invention procedure. The y-axiscontains grain size intervals in millimetres, and the x-axis thepercentage by volume of each fraction. Grain size was measured by laserdiffraction with dry method sample insertion.

FIG. 2.—It shows a representation referring to the micrograph of the Alpowder in granules used in the invention procedure.

FIG. 3.—It shows the graph referring to the standard grain sizedistribution of the Al used in the invention procedure. The y-axiscontains the grain size intervals in millimetres, and the x-axis thepercentage by weight of each fraction. Grain size was measured by asieve tower.

FIG. 4.—It shows a diagrammatic, partially sectional, side elevationalview of the device for the execution of the invention procedure.

FIG. 5.—It shows an elevational view, front and sectional in this case,of the same device as in the previous figure.

PREFERRED EMBODIMENT OF THE INVENTION

The invention procedure, designed to produce Mn minitablets bycompacting, with a concentration of more than 90% of this metal, isbased on using electrolytic Mn ground from flakes of a chemical purityof 99.7% or more. The product is then screened with a sieve with a meshof less than 450 microns, since it has been found that materialscontaining significant fractions of a larger grain size give rise tomuch lower dissolving rates in the aluminium furnace. The grindingprocess is controlled so that the content of Mn fine powders (below 100microns) is more than 15%, as above this percentage it has been foundthat the compacting of minitablets cannot be assured with more than 90%Mn in its composition. FIG. 1 shows the graph referring to the standardgrain size distribution of the Mn used.

The tests made indicate that the Al powder most suited for compacting Mntablets with a concentration of more than 90% is powder atomised bymechanical procedures, the special performance of this AL powder beingdue to its spongy granule structure that permits suitable fluidity onthe metal surfaces of hoppers but which maintains sufficient air holesin the grains, so that the material is endowed with greatercompressibility. FIG. 2 shows the micrograph of the Al powder in grains,according to a microscope enlargement of this type of powder.

The foregoing Al powder also has a controlled grain size distribution,its nominal grain size intervals being between 100 and 800 microns, withover 80% powder between 350 and 720 microns. This grain sizedistribution is coarse enough to enable the material to be compacted andfine enough not to retard the dissolving rate, through having reducedthe number of Al grains (which trigger the dissolving reaction on theminitablet Mn in the furnace) with the increased Mn concentration in theminitablet. FIG. 3 shows the graph referring to the standard Al grainsize distribution in the grains used.

The device for executing the procedure is represented in FIGS. 4 and 5,comprising a hopper (1) for reception and storage of the mix, which isfed in through the respective filler neck (2), a mix which, as stated,is composed of Mn and Al. The mix has to be homogeneous and, on beingreceived in the hopper (1), it falls on a centrally positioned diffuser(3), a diffuser (3) that has a conical layout and is supported on legs(4), so that this diffuser forces the product to flow through the sidesof the hopper (1) and never directly onto the feeder hopper (5) providedat the outlet of the hopper (1), and from which hopper (5) the productmoves onto the compacting hopper (6). The diffuser (3) prevents theeffects of product separation and assures continuous fluidity at thesame level of product in the hopper (1). The compacting hopper (6) is avertical continuation of the feeder hopper (5), so that the formerdefines a chamber which maintains a product level and in which thecompacting is done by means of both fixed punches (7) and moving punches(8).

The compacting hopper (6) is provided with a series of dies (9), ofvarying number depending on the size of the device, and the product orMn and Al powder reaches these dies (9) by way of a honeycomb valve (10)interposed between the feeder hopper (5) and the compacting hopper (6),so that a metered amount of product passes through this valve and isloaded onto each one of the dies (9), as the honeycomb valve (10) formsa sort of drum-sector that is loaded with a given quantity of product sothat, when this valve turns through an angle, the corresponding sectorload discharges on the compacting hopper (6) and the product reaches therespective die (9). The dies are arrayed on a support (11) which isintegral with the actual compacting hopper (6), and that support-hopperassembly is mounted on guides (12), along which it may move in eitherdirection under the action of a pneumatic device, on which guides (12)there is in turn a moving punch (8) support (13) mounted, also driven bya pneumatic ram or device. The support-hopper movement is independent ofthe moving punch movement, although such movements must be synchronisedin order to fill, press, compact and eject the formed minitablet.

The fixed punches (7) are arranged co-axially facing the moving punches(8), the latter being installed on a static support (14).

In this way, when the support (11) with the compacting hopper (6) movesforward, the die (9) is filled and then trips the moving punch (8),which advances and compacts the material located between it and thefixed punch (7). The moving punch (8) then moves back and thesupport-hopper assembly slides slightly forward so that the fixed punch(7) ejects the minitablet, whereupon the cycle starts over again.

It is essential for this device to maintain a minimum product columnlevel in the compacting chamber (6), so that none of the punchesattempts to compact an empty die, which would result in the breakage ofthe punches and column or chamber. This level is maintained by the useof three electrical controls and the afore-mentioned honeycomb valve(10), controls which correspond to references A, B and S, and whichindicate the maximum level, minimum level and safety level of theproduct in the compacting chamber (6), all of this in such a way thatthe safety level S causes the device to shut off if the product dropsbelow this level because there will be a risk of emptying the chamber,whereas level B is the product level that permits a reproducible columnweight to be maintained capable of assuring suitable fluidity andconsistent reproducible filling at all the punches. When the product hasreached that level, the honeycomb valve (10) opens and dispenses moreproduct from the hopper. This honeycomb valve (10) closes when theproduct reaches the maximum level A.

To obtain proper compacting of the half-tablet with an Mn concentrationof 90% or more, it is necessary to work with punches capable of applyinga pressure of 7500 Kg/cm² of punch. In a practical example a check wasmade on the mechanical strength of the product obtained with 90% and 95%Mn, in the conditions explained, a mechanical strength check that wascarried out by means of a drop test consisting of dropping a number ofminitablets onto a cement floor from a height of 1 m, recording thenumber of impacts required to cause breakage and for the loss of 2%weight of the minitablet. Minis Mn 90% Minis Mn 95% Number of tests 5 5Drops to 2% weight loss 3 ± 1 1.3 ± 0.6 Drops to breakage 3.7 ± 0.6 2.3± 0.3

Dissolving tests of these Mn minitablets with concentrations of 90% ormore were conducted in Al baths, using for this purpose a rotarygas-fired semi-industrial furnace with a capacity of 400 kg Al. Theexperiments were performed in accordance with regular standard processesfor the addition of minitablets, bath slag removal, stirring and samplecollection. The samples were analysed by spark spectrophotometry.

1. A method for the manufacture of high concentration manganeseminitablets for aluminum bath alloying which, having as its object theproduction of Mn minitablets or tablets with a concentration between 90and 98% of said metal, starting from a mixture of powdered Mn Al, forthe alloying of aluminum and other metal baths, which comprises usingelectrolytic Mn ground from flakes of a chemical purity of 99.7% ormore, and Al powder atomized by mechanical means, with a controlledgrain size distribution between 100 and 800 microns, and with over 80%powder between 350 and 720 microns, while a check is made on the Mngrinding such that a content of fine Mn powder with a size of less than100 microns, is not more than 15%.
 2. The method for the manufacture ofhigh concentration manganese minitablets for aluminum bath alloying,according to claim 1, characterized in that the ground electrolytic Mnis subjected to a screening process with a sieve with a mesh of lessthan 450 microns.
 3. The method for the manufacture of highconcentration manganese minitablets for aluminum bath alloying,according to claim 1, characterized in that the level of the Mn and Al,mix in the corresponding compacting means monitored by respectivesensors to keep this mix level between limits that assure the executionof the actual compacting.
 4. A device for the manufacture of highconcentration Mn minitablets for aluminum bath alloying, designed forthe execution of the procedure of claim 1 starting from a mixture ofground electrolytic Mn powder and atomized Al powder, said devicecomprising: a mix storage and reception hopper; a compacting means in asuitable compaction chamber with dies in which the minitablets areformed, comprising punches both for pressing and ejecting the formedtablets, wherein the storage hopper is provided with a central diffuserthat diverts the product towards the sides of the hopper, therebypreventing said product from passing directly to the respective feederand compacting chambers, including between the feeder hopper and thecompacting hopper; and a honeycomb valve for dispensing product to therespective dies forming part of the compacting chamber in which theminitablets are formed; said honeycomb valve being designed to bedivided sectorally so as to go on supplying the doses of product to thecompacting chamber individually, with the result that each of the diesis filled for subsequent compacting, formation of the tablets and theirejection by means of the respective fixed punches, which act incombination with other moving punches to carry out the compacting andpressing of the product in the dies.
 5. The device for the manufactureof high concentration Mn minitablets for aluminum bath alloying,according to claim 4, further comprising three electrical product levelsensors in the compacting chamber to monitor the maximum level A, theminimum level B and the safety level S, which determine the correctcompacting of the product in the dies.